Lithium ion secondary batteries are structured, for example, to have an electric storage element and an electrolyte solution housed in an exterior, where the electric storage element is configured such that a positive electrode formed by applying an active material (lithium composite oxide) for positive electrodes to a sheet-like collector foil (such as aluminum foil or copper foil) and a negative electrode formed by applying an active material (such as activated carbon or carbon) are laminated with a separator interposed therebetween for preventing short circuit by contact between the positive and negative electrodes.
As such a battery, Patent Document 1 proposes a non-aqueous electrolyte battery configured to have a ceramic separator layer 111 with the use of, not a separator such as a polyolefin stretched film (hereinafter, referred to as a “polyolefin separator”) used conventionally, but a separator with inorganic microparticles dispersed in an organic polymer (hereinafter, also referred to as “a ceramic separator layer”), the ceramic separator layer 111 being disposed between a positive electrode 101 and a negative electrode 102 as schematically illustrated in FIG. 4.
The ceramic separator layer 111 used in Patent Document 1 is not shrunk by deformation even at high temperatures. Therefore, even when the ceramic separator layers 111 are exposed to unintended high temperatures, safety can be improved without causing short circuit between the positive electrode and the negative electrode, heat generation, smoke generation, ignition, or the like by shrinkage of the layer. For example, the safety which causes no ignition even in a nail penetration test is due to this feature.
However, there is actually no launched battery configured such that electron insulation is ensured by only the ceramic separator layer 111, because electron insulation is not able to be ensured by only the ceramic separator layer in the case of adopting an electrode with large surface asperity.
Moreover, when the ceramic separator layer is adopted to try to respond to requests for low cost and low resistance, the film thickness has to be reduced, and in the case of adopting an electrode with large surface asperity, electron insulation is not able to be ensured when the ceramic separator layer is reduced in thickness. On the other hand, while electron insulation can be ensured when the film thickness of the ceramic separator layer is increased in order to ensure electron insulation, the increased film thickness has the problem of leading to an increase in cost and an increase in resistance. As just described, low cost, low resistance, and electron insulation are actually not all satisfied in the case of the configuration shown in FIG. 4.
Furthermore, Patent Document 2 proposes, as schematically illustrated in FIG. 5, a non-aqueous electrolyte battery configured to have a porous insulating layer (HRL) (substantial ceramic separator layer) 111 and a porous insulator (commonly used polyolefin separator) 112 provided between a positive electrode 101 and a negative electrode 102.
It is to be noted that the porous insulating layer 111 is formed from a mixture of insulating inorganic microparticles and a binder composed of an organic polymer, and substantially the same as the ceramic separator layer.
In the case of the configuration in Patent Document 2, the porous insulator 112 which is a polyolefin separator and the porous insulating layer (ceramic separator layer) 111 composed of a mixture of insulating inorganic microparticles and a binder of an organic polymer are interposed between the positive electrode 101 and the negative electrode 102, and short circuit between the positive electrode and the negative electrode, heat generation, and ignition are suppressed or prevented for the improvement of safety by interposing the porous insulating layer (ceramic separator layer) 111 which is not shrunk at high temperatures, while electron insulation between the positive and negative electrodes can be ensured by the porous insulator 112 which is a polyolefin separator with excellent electron insulation. However, there are the following problems because the porous insulator 112 which is a polyolefin separator is used in combination.
(a) The cost of the polyolefin separator accounts for a large percentage of the battery cost, and causes an increase in cost.
(b) The polyolefin separator is high in resistance, thereby resulting in a deterioration in power characteristics, and the reduced film thickness and the increased porosity are thus conceivable as countermeasures against the deterioration, but not easily achieved, and become strong factors in preventing high performance of the battery. Moreover, in order to ensure power characteristics, the increased number of layers stacked is conceivable, which results in an increase in cost.
(c) The polyolefin separator typically has a large film thickness of 20 to 30 μm, and has the problem of decrease in energy density per volume, and it is extremely difficult to reduce the film thickness of the polyolefin separator because of problems in handling, etc., although a higher energy density battery can be designed as the separator is reduced in film thickness.
Furthermore, Patent Document 3 proposes, as schematically illustrated in FIG. 6, a lithium ion secondary battery including: (a) a first insulating layer (ion-permeable gel) 113; (b) a second insulating layer (ceramic separator layer that has ion permeability) 111; and (c) a porous insulator (porous polyolefin separator) 112 between a positive electrode 101 and a negative electrode 102.
In the case of the configuration in Patent Document 3 herein, the problems mentioned above with Patent Document 2 are not only directly applied thereto because there are the ceramic separator layer (second insulating layer) 111 and porous polyolefin separator (porous insulator) 112 provided therein, but problems such as high cost, high resistance, a decrease in energy density, and a decrease in power density are also further grown because of the further addition of one more constituent element referred to as the ion-permeable gel (first insulating layer) 113.
Besides, proposed are: a non-aqueous electrolyte secondary battery in which a porous protective film of 0.1 to 200 μm in thickness, composed of a resin binder and solid particles, for example, is formed on the surface of either a negative electrode active material coating layer or a positive electrode active material coating layer (see Patent Document 4); a non-aqueous electrolyte secondary battery in which a negative electrode is provided with an active material layer including particles of an active material containing Si or Sn, and a layer including particles of an inorganic oxide is formed on the outermost surface; and further, a non-aqueous electrolyte secondary battery with outermost surface roughness Ra as specified in JIS B0601 from 0.1 to 3 μm (see Patent Document 5).
However, also in the case of the batteries in Patent Documents 4 and 5, porous polyolefin separators are used as the separators, and there are actually such problems as mentioned above with reference to Patent Documents 2 and 3.
Patent Document 1: Japanese Patent Application Laid-Open No. 2006-164761
Patent Document 2: International Publication WO 2005/098997
Patent Document 3: Japanese Patent Application Laid-Open No. 2010-267475
Patent Document 4: Japanese Patent Application Laid-Open No. 7-220759
Patent Document 5: Japanese Patent Application Laid-Open No. 2009-164014